1. Field of the Invention
The present invention relates to bulk shrinkable thermoplastic packaging films particularly suitable for bundling unit packs of individual articles to be packaged, such as canned foods, other cans, bottles or the like, as well as to a method for producing these films and to the final product packaged items, per se.
2. Description of the Prior Art
For packaging and storing groups of times of widely varying types, thermoplastic films have long been used in lieu of the conventional cardboard boxes for bundling a plurality of identical or different articles to form a unitary pack. It is thus common to, for example, find packs of very many different types of articles on the market such as canned food, bottles, or cans in a packaging consisting only of shrinkable plastics film.
Such unitary packs are provided in known manner by bundling. Bundling is a technique which entails enveloping the totality of the items to be packaged with a shrinkable thermoplastic reins film which can be welded to itself such as to form a sleeve around the grouped items. The film is then shrunk by, for example, transferring the entire assembly through an oven at a temperature that permits the thermoplastic resin constituting the film to soften, thus relieving internal stresses. Upon exiting the oven, rapid cooling ensures that the film shrinks tightly and sealedly around the items contained therein. Thus, a highly homogeneous bundle or unitary pack is produced in which the thermoplastic film functions as a skin in tight contact with the surface of the packaged items.
All thermoplastic polymers or copolymers, in the form of shrinkable films, having a sufficient tear resistance can be employed for this type of application. Nevertheless, the polyolefins and, more particularly, polyethylene or polypropylene, or even copolymers of ethylene and propylene and an olefinically unsaturated monomer such as vinyl acetate, are those that are most frequently used industrially.
The shrinkability and tear resistance properties required of such thermoplastic films present, in the case of known films, certain disadvantages when the packaged items are being unpacked or removed. In particular, it is necessary, in light of the fact that the film is difficult to tear, to exert a quite significant force in order to stretch the film packaging to provide a vehicle for removing the items contained in the package. It may even sometimes be necessary to employ a cutting tool, which is dangerous, particularly in the home, with concomitant risk of damaging or destroying the packaged items.
If the packaging film is indeed torn, the tear is quite random and cannot be controlled, often resulting in the items falling out of the pack or, yet again, the tearing is in the direction of extrusion of the film. This latter property presents no advantages or interest whatsoever. In effect, shrinkable thermoplastic films employed in this packaging application are produced by extrusion, followed by winding onto a large roll. When the film is actually used, the items are packaged in the direction of the length of the film as it progressively unwinds from the roll. Under these conditions, the tear properties of the film in the direction of extrusion in other words along the length of the film or "machine direction", do not in any way facilitate the subsequent complete opening of the package.
U.S. Pat. Re. 30,726 describes blow extrusion into films of mixtures of polyethylene and ionomer resins. Such films have tear properties in the extrusion direction, in other words in the direction of the production machine. This film suffers from the disadvantages indicated above. Such a film can only be employed for packaging items of small size, e.g., packets of chewing-gum or cigarettes, and cannot be employed for the continuous bundling of larger objects such as bottles, unitary packs, etc. Moreover, during handling of larger size articles, the purchaser grasps the packaging by the crescent-shaped opening provided in the sides of the pack. In the event that the film possesses tear properties in the direction of the production machine, the packaging tears when it is handled by these openings. Hence, a serious need continues to exist in the packaging industry to provide a film which can be readily torn, but which retains the necessary strength for bundling.